Vehicle suspension systems are important for both maintaining vehicle control and for isolating a vehicle's contents from bumps and vibrations caused by imperfections in the road. Typical suspension systems may utilize one or more of springs, shock absorbers (or dampers), hydraulic cylinders and air bags.
Isolating the vehicle's contents from bumps and vibrations caused by road imperfections allows the vehicle to ride relatively undisturbed. Suspension systems are designed to absorb the energy generated when the vehicle rides over unevenness on the road and to dissipate the energy without causing undue vertical movement of the vehicle body.
A proper suspension system also assists in maintaining vehicle control by minimizing weight transfer of the vehicle from side to side and from front to back that occurs during movement, braking and acceleration. This allows all of the wheels to better maintain contact with the ground, and maintains the vehicle's ability to steer, brake, and accelerate.
The present invention relates to towed vehicles used to carry a payload such as trailers, as well as to their load-sharing accessory vehicles known as tag dollies and boosters. For simplicity, the term “dollies” will be used to refer collectively to all of such accessory vehicles.
An effective dolly suspension system may help to prevent “bridging”, a condition that occurs when the wheels from one or more axles unload, or lose contact with the ground, typically caused by uneven road conditions. As a result, the entire load is borne by the remaining axles which may damage the dolly, and cause reduced brake performance or even loss of control if the axles are self-steering.
Air bag suspension systems have an “air bag performance zone” defined by the manufacturer. The air bag performance zone determines the preferred range of ride heights, defined as the distance between axle and the frame. It is important that the suspension system maintain a ride height within this preferred ride height range in order to provide the most effective suspension and to ensure optimal performance and operating life for the air bags. One prior art suspension system for a dolly includes both a hydraulic bias mechanism (comprising one or more hydraulic cylinders) and an air suspension mechanism (comprising one or more air bags). In normal operation, the hydraulic cylinders are fully extended thereby acting effectively as struts between the towing vehicle and the dolly to bias the dolly frame toward the ground. The air bags provide the suspension performance. In the event that the air bags threaten to collapse, the cylinders are immediately retracted to reduce the downward bias of the dolly and to relieve the air bags. The operator of the dolly manually adjusts the suspension system upon loading or unloading cargo so that the ride height falls within the preferred range by removing or inserting metal shims in the hydraulic bias system so as to increase or decrease the downward bias. The manual removal and insertion of shims according to the payload is time-consuming.
The distribution of the load can also be affected by a significant change in road conditions, such as an abrupt change in road grade or cornering an elevated curve. This change in distribution can affect the ride height and create handling problems.
When the dolly is traveling, variations on the surface of the road may affect the ride height as well. For example, if the dolly encounters a bump on the road, the wheel (and axle) will experience upward vertical acceleration and rise, resulting in the compression of the air bags and instantaneous variations in the ride height that are normally compensated for by the air suspension system. If the bumps on the road are extreme, the air suspension system may be driven outside the air bag performance zone, bottom out and cease to provide suspension. This condition has a negative impact on the handling of the dolly and may lead to damage to the suspension and to the dolly. In prior art suspension systems that use a hydraulic bias system to achieve a nominal ride height, when the air suspension mechanism has collapsed, the hydraulic bias mechanism is activated on an emergency basis to decrease the load on the air suspension. This usually results in a violent jolt to the dolly, which is uncomfortable and potentially damaging to the cargo.
It is therefore an object of the present invention to provide a suspension system that overcomes the aforementioned limitations.
The particular objects of the invention will be better understood by reference to the detailed description of the preferred embodiment that follows.